In recent months a couple of users of Hacker News have walked away from the site. First jacquesm said he was leaving and then RiderOfGiraffes decided to quit also.
Yet, neither seems to have totally left: jacquesm took a break and started posting articles after a couple of months and has participated in some discussions (although not as actively as before).
RiderOfGiraffes reappeared in the guise of ColinWright (which I only spotted because ColinWright posted a link to one of RiderOfGiraffes submissions revealing the link).
I even took a break from Hacker News a while back.
What is it that makes Hacker News so hard to leave? The answer: community.
The community has managed to stay on topic, spam-free and courteous over a very long period. I've been a member for four years (I joined when Hacker News was about 4 months old) and although there are periodic complaints about the quality of Hacker News declining they are merely complaints. No other site I've been involved in (and I'm including going all the way back to BBS and Usenet days) has managed to keep its community civil, interesting and active for that long.
There's a lot of talk about community in web forums, but only Hacker News has managed to keep it together. Reddit, for example, has turned into what Digg should have been, and even the Programming subreddit has withered.
On Hacker News it's common to hear people say they read the comments before reading the article. That's the acid test. The comments are good because the community is strong.
Above all, Hacker News has kept the signal-to-noise ratio high. Primarily, I believe this is because, like any good open source project, Hacker News has a BDFL. Paul Graham has stepped in with code and requests for civility that have kept the community on track. With his guidance others have followed suit and kept the community regulated.
Hacker News has also been successful, IMHO, because it's followed a "don't worry, be crappy" philosophy of only adding features when totally necessary. For example, there was no search function for years (literally!), you don't get notification of when someone replies to a comment, the site's design is Craigslist-bare, you can't collapse threads, etc. etc. By focussing on what people write, and not the bells and whistles, Hacker News has been successful.
(Now, if only I can do the same for jobs on UseTheSource).